Polyalkylene terephthalates have acquired considerable importance as raw materials for the production of fibers, films and molded articles. Because of their partially crystalline structure, they have outstanding properties, such as high abrasion resistance, favorable creep characteristics and high dimensional accuracy, and are therefore particularly suitable for the production of moldings subjected to severe mechanical and thermal conditions. An additional improvement in the mechanical properties can be achieved by incorporating reinforcing materials, for example, glass fibers (British Patent Specification No. 1,111,012; U.S. Pat. No. 3,368,995 and DT-AS (German published specification) No. 2,042,447).
Because of its special physical properties, polyethylene terephthalate is particularly suitable for the production of fiber products and films. However, the need for high mold temperatures (about 140.degree. C.) and relatively long press times is a disadvantage in the production of molded articles, which is only partially compensated by extraordinary rigidity and high heat stability under load. Polypropylene terephthalate and polybutylene terephthalate indeed require shorter press times and lower mold temperatures (about 100.degree. C.) than polyethylene terephthalate, since they crystallize considerably more rapidly, but compared with polyethylene terephthalate they have poorer physical properties, in particular a lower heat stability under load.
There has been no lack of endeavors to provide polycondensates which have a combination of the good properties both of polyethylene terephthalate and of polypropylene terephthalate and polybutylene terephthalate. Thus, it is known, for example, that the tendency towards crystallization of polyethylene terephthalate can be improved by nucleation with finely divided, solid inorganic substances (Dutch patent specification No. 6,511,744).
Highly crystalline, thermoplastic terephthalic acid copolyesters which crystallize rapidly and consist of at least 90 mol %, relative to the dicarboxylic acid component, of terephthalic acid radicals, 90 to 99.5 mol %, relative to the diol component, of ethylene glycol radicals and 0.5 to 10 mol %, relative to the diol component, of co-diol radicals, characterized in that (a) the co-diols employed carry two primary OH groups, (b) these two OH groups are separated by 3 or 4 C atoms and (c) the co-diols employed are either unsubstituted, mono-alkyl-substituted or dialkyl-substituted, the alkyl groups carrying 2 or 3 C atoms and the sum of the C atoms of the alkyl substituents being at least 4 in the case of substitution, are known from DT-OS (German published specification) No. 2,507,674 and U.S. patent application Ser. No. 658,816, filed Feb. 17, 1976.
Examples of suitable co-diols which may be mentioned are propane-1,3-diol, butane-1,4-diol and 2,2-diethylpropane-1,3-diol.
Highly crystalline, thermoplastic terephthalic acid copolyesters which crystallize rapidly and consist of at least 90 mol %, relative to the dicarboxylic acid component, of terephthalic acid radicals, 90 to 99.5 mol %, relative to the diol component, of ethylene glycol radicals and 0.5 to 10 mol %, relative to the diol component, of co-diol radicals, characterized in that (a) the co-diols employed carry at least one secondary or tertiary OH group, (b) the co-diols employed contain 4 to 10 C atoms and (c) the two OH groups are separated by 3 to 4 C atoms, are known from DT-OS (German published specification) No. 2,507,776 and U.S. patent application Ser. No. 658,817, filed Feb. 17, 1976.
Examples of suitable co-diols which may be mentioned are, in particular, butane-1,3-diol, hexane-2,5-diol, 2,2,4-trimethylpentane-1,3-diol and 2- and 3-methylpentane-2,4-diol.
Both the German Offenlegungsschriften (German Published Specifications) cited disclose that the tendency towards crystallization of the polyethylene terephthalates is not reduced by incorporating the specifically defined co-diols, but is significantly increased.